In 1800, astronomer Sir William Herschel discovered which type of invisible radiation beyond the red end of the visible spectrum while studying sunlight with a prism?

Introduction / Context:
The electromagnetic spectrum includes many types of radiation beyond the range of visible light. A key historical discovery was made by Sir William Herschel in 1800, when he detected invisible radiation while examining the heating effect of sunlight. This question asks you to recall which specific type of radiation he discovered beyond the red end of the visible spectrum, a foundational moment in physics and astronomy.

Given Data / Assumptions:

• The experiment occurred in 1800.
• Herschel used a prism to disperse sunlight into a spectrum.
• He measured temperature in different colors and beyond the visible red.
• We are choosing among known types of electromagnetic radiation.

Concept / Approach:
Herschel passed sunlight through a glass prism and placed thermometers in different parts of the spectrum. He noticed that the temperature increased from blue to red and was even higher just beyond the red region where no visible light was seen. He concluded that there must be an invisible form of radiation there, which is now called infrared radiation. To answer this question, we must connect his heating experiment with the discovery of infrared, and distinguish that from ultraviolet, X-rays, or microwaves, which were identified later by other scientists.

Step-by-Step Solution:

Verification / Alternative check:
Physics references and educational websites routinely attribute the discovery of infrared radiation to Herschel in 1800. They describe how he used thermometers and a prism to show that invisible radiation beyond red carries heat. Ultraviolet radiation beyond violet was discovered shortly afterward by Johann Wilhelm Ritter, and X-rays and microwaves came even later. Since Herschel is specifically linked to infrared, checking basic timelines confirms that "Infra-red radiation" is the only accurate option.

Why Other Options Are Wrong:
Ultraviolet rays lie beyond the violet end of the visible spectrum and were not discovered by Herschel. X-rays are very high-energy radiation discovered by Wilhelm Röntgen in the late 19th century. Microwaves are lower-frequency radio waves that were studied much later in the 20th century. None of these match Herschel's 1800 experiment with temperatures just beyond the red part of sunlight, so they are incorrect choices for this specific historical question.

Common Pitfalls:
A frequent mistake is to confuse "infrared" and "ultraviolet" because both are invisible and appear close to the edges of the visible spectrum. Some learners may also forget which side of the spectrum each belongs to. To avoid this, remember the order: going from shorter to longer wavelengths, you have ultraviolet, visible, and then infrared. Herschel detected extra heat beyond red, which clearly points to infrared radiation rather than ultraviolet or any higher-energy radiation like X-rays.

Final Answer:
In 1800, Sir William Herschel discovered infra-red radiation beyond the red end of the visible spectrum.